For our 3 youngest kids (2-6 years), we use the plastic (polyethylene?) kids rainsuits that Gander Mountain sells for around $10. For the oldest (almost 9, tall for age), size small DriDucks works, but they're big on her. Bring duct tape for patching.

I'd get her a cheap plastic poncho (not the really flimsy emergency ponchos though). A plastic poncho should cost about $10 and weigh maybe 10oz. I'd get one that's a bit on the big side for the kids so they're covered more. You can tie a peice of cord around their waist to make it less awkward for her.

If kids are playing in the rain long enough the WILL get wet no matter what kind of rain gear they have. The poncho or rain suit will not keep them perfectly dry but it will slow the wetting process down so their body can warm up the water that makes its way through. Being wet under a rain suit or poncho is MUCH warmer than just getting soaked, trust me. Depending on how much rain you expect I'd suggest at least a fleece for your girl and possibly an entire set of dry clothes for after the rain is over.

Check REI, especially when on sale. Also check Campmor.com, although their sizes seem to run big. Actually, it's no big deal if you get them a little too big; it may take a bit longer to outgrow them! The pant legs and sleeves can be rolled up in the meantime. If they're a bit big, there's more ventilation inside. Just be sure there's enough room for the warm puffy jacket inside the rain jacket!

My son found a great rain jacket for his 12-year-old at Second Ascent in Seattle. Very slightly used. Of course with three kids, the outgrown stuff gets handed down to the next sibling. Just don't buy pink if the girl is the middle one of the three! The youngest boy (only 4 at the time) was willing, but his father most definitely wasn't!

The 4-year-old should be able to carry a small pack with her own wraps, a snack and a pint water bottle. The whole thing should be no more than 3-4 lbs. and she'll be proud to think she's carrying her own stuff!

Edited to correct name of store; Next Adventure is the one in Portland and I keep mixing them up.

Seems like a hoodless poncho would be a good quick and dirty MYOG project. Throw in a rain hat and you're ready to go. If you want to get more mileage, make it longer and add snaps/velcro to adjust it as they grow. Make it longer in the back to cover their pack.